FIG. 9 is a sectional view in which a contact portion in a conventional vacuum interrupter is enlarged. This contact 1 is arranged in a vacuum vessel and is formed with a plurality of spiral grooves, each of which smoothly changes its direction from a center portion to a rim portion. In this drawing, a reinforcement plate 5 and a spacer 8 are arranged on the back of the contact 1; and an electrode rod 6 is joined to the contact 1 via the reinforcement plate 5 and the spacer 8.
In such a spiral shaped contact 1, when fault current is interrupted, current flows along the contact 1 machined in the spiral shape and thereby generating a magnetic field in a radial direction, and a concentrated arc due to the fault current generated between the contacts 1 is driven in a circumferential direction by the magnetic field, whereby the arc is prevented from remaining in a certain place of the contact 1 and interruption performance is improved. The reinforcement plate 5 prevents metallic vapor or the like generated between the contacts 1 during the interruption of fault current from dispersing to the backside of the contact 1, from attaching to the inner surface of ceramics, and from degrading withstand voltage performance.
Furthermore, generally, contact pressure is exerted between the contacts 1 in a contact closed state; and force exceeding the contact pressure is further temporarily generated at a moment when the contacts 1 collide during contact closing operation, a lot of stress is generated in the contact 1, and the contact 1 is likely to be deformed. The reinforcement plate 5 also combines the role of reinforcing so that the contact 1 does not deform by being arranged on the backside of the contact 1 while coming in contact with the contact 1.
Material such as stainless steel, which is stronger in strength and higher in resistance than the contact 1, is generally used for material of the reinforcement plate 5; however, current is shunted to the reinforcement plate 5 according to the resistance ratio between the contact 1 and the reinforcement plate 5. The spiral shaped groove is not formed in the reinforcement plate 5; and accordingly, a magnetic field is not generated from the current that flows through the reinforcement plate 5, the magnetic field generated from the contact 1 is reduced by the amount of current that flows through the reinforcement plate 5 and it causes to degrade the interruption performance.
For example, if the structure is such that a contact does not come in contact with a reinforcement plate behind the contact as in U.S. Pat. No. 8,039,771, current that flows through the reinforcement plate disappears and a magnetic field generated from the contact increases, but a function serving as the reinforcement of the contact also disappears and the contact is more likely to be deformed.
More particularly, in the case of a contact for use at a high contact pressure and a shape in which a recess at a central portion of the contact is large and which comes in contact at a rim portion as in JP,3812711,B, a problem exists in that deformation of the contact increases and adoption of such a structure is difficult.